The present invention relates first of all to a process which allows the colorimetric and possibly dimensional measurement of a coloured surface and to the processing of the information measured on the said surface or on part of the latter.
The present invention also relates to the device for the calorimetric measurement of the said surface.
Many calorimetric measurement-taking devices are known which make it possible to give, by tristimulus measurement, values of the three chromatic parameters, R (red), G (green) and B (blue), of a calorimetric measurement. For this purpose, an illuminant is used so as to illuminate a surface on which the measurement will be taken. It should be pointed out that this measurement is normally a measurement of the average chromatic values of the illuminated surface.
Patent Application FR-A-2,749,077 has proposed a method and a device for measuring the colour by calculating the trichromatic components with respect to a colorimetric reference system. This calculation is made using a matrix for transferring the calorimetric system associated with the data acquisition system to the calorimetric reference system. The said transfer matrix is calculated by an iterative procedure based on the trichromatic components measured by the calorimetric reference system of the three primary colours. However, it is found that the measurements are taken using a tri-CCD camera having three CCD sensorsxe2x80x94one CCD sensor for each of the three trichromatic components. The cost of this camera is relatively high. Furthermore, it is found that the illuminant is placed outside the acquisition system including the camera. This means that perturbations due to the surrounding (ambient) illumination will be picked up during measurement by the acquisition system. Furthermore, it is found that no dimensional or spatial processing of the image is possible with the system described in this publication.
Document U.S. Pat. No. 5,724,259 describes a method for monitoring the colour of a printed image on a substrate, which makes it possible, using a camera, to measure the light reflected from the said printed image onto the substrate, to transform this measurement into a signal sent to a computer and to use the information emanating from this signal to correct the effects of scattered light reflected from the printed image. The main use of this method is therefore to check the ink thickness in the case of applications of printing machines such as printers. For this purpose, the optical density defined by the logarithm to the base 10 of the reflectance, this being a percentage of the scattering of the incident light, is more specifically measured. For this purpose, a reference surface not incorporated into the apparatus is placed beside the printed image to be measured. This also means that the measurement is not a contact measurement but indeed a remote measurement.
Document EP-A-0,491,131 describes an apparatus for monitoring and calibrating the spectrum emitted by a coloured object using the dispersion of the wavelength provided by a variable filter. This is a calibrating apparatus working directly as a calorimeter. However, in the present case, because of the presence of a diffuser and a variable filter between the one-dimensional sensor and the object itself, it is estimated that no spatial information could be obtained by the use of such a device.
Document EP-A-0,314,312 describes a method and an apparatus making it possible to detect the presence of ink on a substrate using several light-emitting diodes (LEDs) of different wavelengths which are switched on and off independently of each other.
Document U.S. Pat. No. 3,597,094 describes a portable device allowing colour identification, which works directly with visual observation. This device comprises a plurality of filters positioned between a light source and the coloured surface.
One object of the present invention is to propose an improved method and an improved device which make it possible to obtain a calorimetric and dimensional (or spatial) measurement of a defined region on an image of a surface, which is possibly inhomogeneous, such as dot-matrix printing or a defect appearing on a captured surface.
This will allow a reproducible and reliable measurement of the chromatic parameters of the said defined region to be obtained.
The object of the present invention is also to propose a device and a method which make it possible to take stabilized measurements with respect to a reference standard.
The object of the present invention is also to allow the use of this information for medical, industrial, cosmetic and food applications or else in any other application using the technique of colorimetry.
The present invention relates first of all to a method for the calorimetric and possibly dimensional measurement of a defined region on an image representing a surface. The method consists in taking an image of the said surface using a colour camera, in transforming and converting the analogue information measured on the said surface into digital form, preferably using a processing unit on board of the colour camera or in a computer, in delimiting the said defined region on the said image and in carrying out the measurement of the chromatic values R, G, B for this region.
The term xe2x80x9csurfacexe2x80x9d should be understood to mean the surface for which it is desired to obtain colorimetric-type and/or dimensional-type information.
The term xe2x80x9cimagexe2x80x9d should be understood to mean a group of pixels which represents the captured surface and which may be directly transmitted to a monitor such as a screen or a printer.
The term xe2x80x9cregionxe2x80x9d should be understood to mean that part of the said image for which the calorimetric and/or dimensional measurement is carried out.
Optionally, the method also consists in measuring the distance between two pixels of the said image or in delimiting the included area during peripheral tracing of a region of the said image.
The definition of the said region may be carried out either manually by peripheral tracing, or automatically by scanning the various pixels of the image for which the chromatic parameters fall within a predetermined window or interval.
The present invention also relates to the device for implementing the said method, comprising a sensor provided with a colour camera and with an illuminant which are placed within the same enclosure associated with an analogue information acquisition system, the information being measured using the said sensor and being converted into digital form by a processing unit.
Preferably, the colour camera is a mono-CCD or mono-CMOS camera.
Preferably, the illuminant will be produced using a single, white or colour, diode or several identical, white or colour, diodes having the same spectral distribution.
According to a preferred embodiment, the sensor furthermore includes, within the same enclosure of the sensor, a reference standard placed in the field of view of the said camera. This makes it possible to stabilize each measurement.
According to a preferred embodiment, the information converted by the processing unit is transmitted to a monitor which may be a screen or a printer and which gives a precise display of the image. Optionally, using this monitor, the image may also be processed so as to measure the distance between two specific points or pixels, to measure the area of the said region, etc.
The data will be obtained by a tristimulus measurement of the R, G, B parameters, preferably by matrix computation.